As much as I agree with the first part of your statement, I don't reach the same conclusion. Perhaps you don't know the full story. Apple itself admitted that there is a portion of the Pro market which finds an AIO suitable for their workflow. After all, there are people happy about the tcMP. You are just the other portion which will be addressed by the mMP, like many of us.
Think about it. At the end of the day, you are at the border between the two groups, since you stated you want to compare the machines first, before buying one of them.
Finally: 4.999$, not 6k. And the value of the parts is quite worth it, although I do agree that it doesn't suit the needs of some professionals.
I think you have hit the nail on the head here. You can't please 100% of the people 100% of the time, there are going to be professionals that will love this iMac and do in fact love the iMac as an all in one and those are the people Apple is going after with the iMac Pro, the other professionals who want something different are the reason why Apple are working on a new Mac Pro.
As for price I think people forget that these professionals will not have a problem paying for the power, also your getting a great 5K display.